by Cecilie Wettre, Jochen Kerkmann, Gordon Bridge (EUMETSAT) and Jarno Schipper (ZAMG)Jump to images
On Sunday 9 October 2005 Tropical Storm Vince developed a few hundred kilometers northwest of Madeira island, and was upgraded to a hurricane later that day. Although rather small in horizontal dimension, it was classified as a category 1 hurricane with a maximum speed of 125 km/h. Vince caught forecasters by surprise, since the waters in the area are some degrees colder than the 26.5ºC typically needed for a tropical storm. Hurricane Vince gradually weakened as it moved over cooler waters, and was classified as an extratropical cyclone the next day. Vince made landfall on the Iberian peninsula near Huelva, Spain at 09:00 UTC on the 11th of October, and was downgraded to a tropical depression. Vince was the first tropical cyclone on record to make landfall in Spain.
Warm water (or, more specifically, the moisture in the air above it) is the energy source for tropical cyclones. A sea surface temperature above 26.5 degrees Celsius to at least a depth of 50 meters is one of the conditions normally required to make tropical cyclone formation possible. Vince is unusual in that it formed over significantly cooler water.
Tropical cyclones do occasionally form even where sea temperatures do not meet the above-mentioned condition, but only when associated with specific weather disturbances. One such disturbance is an upper tropospheric trough, or a cold-core upper level low. A tropical cyclone may then develop when the upper-level disturbance produces deep convection, which becomes organised.
On the airmass RGB image and animation (below, lower right), the narrow, upper tropospheric trough can be seen as a red contour around the "mini" hurricane, indicating descending stratospheric air with high potential vorticity. Hurricane Vince at its peak lies exactly in the axis of this trough and it looks like it is driven by strong shear vorticity. In addition, it is pictured and surrounded by bluish colours indicating cold, polar airmasses. Finally, a secondary smaller centre of vorticity can be seen running around the southern side of the trough.
Note: The airmass RGB (lower right image) shows not only the height of the clouds, but also the airmass characteristics in cloud-free areas. The WV6.2 channel shows the Upper Tropospheric Humidity (UTH), the WV6.2-WV7.3 difference shows the vertical distribution of humidity and the IR9.7-IR10.8 difference is related to the total ozone content and the height of the tropopause.
Combined together in a RGB, the result is an image where high clouds appear in white colour, mid-level clouds in light ochre colour and cloud-frees areas in dark green colour (warm air mass with high tropopause) or blue colour (cold air mass with low tropopause). A particular feature of this RGB is that the dry descending stratospheric air is marked by a reddish colour.
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